CopolymerizationChapter 9

Copolymerization

Chapter 9

Ch 9 Sl 2

Step copolym’n



polym’n of (ARB + AR’B) or (RA

+ R’B

+ R”B

)

 gives copolymers

 with composition similar to monomer composition

 polym’n to p ≈ 1



sequence distribution?

 with functional groups of the same reactivity

 eg, HOOC-R-NH₂ + HOOC-R’-NH₂

 random copolymer formed

 with functional groups of different reactivity

 eg, A = -COOH, B = -NH₂, C = -OH

 eg, B = -CH₂OH vs B = -CHROH

 ‘blocky’ structure

Ch 9 Sl 3

Chain copolym’n



4 types of copolym’n reaction

 assumption: Reactivity of active center depends only on terminal monomer unit (--A* and --B*).



rate of copolym’n

cross-propagation homopropagation

Ch 9 Sl 4

Copolymer composition equation



copolymer composition = d[A]/d[B]

 steady-state approx

 Rates of cross-propagation are the same.

copolymer composition eqn [Mayo(-Lewis) eqn]

monomer reactivity ratio (homo/cross)

Ch 9 Sl 5

 Copolymer composition depends on r and conc’n of monomers ([A] & [B]) at an instant.

 r depends

 on type of active center (, –, or +)

 on temperature (a little in radical, much in ionic)

 not on initiator, solvent in radical

 on initiator [counter-ion], solvent in ionic



mole fractions

 f_A, f_B in the feed: f_A = [A]/([A]+[B])

 F_A, F_B in the copolymer: F_A = d[A]/(d[A]+d[B])

 (another form of) copolymer composition eqn

Ch 9 Sl 6

(Chain) copolym’n behavior



r determines composition and sequence distribution

 r > 1 ~ prefer to homopolymerize r < 1 ~ prefer to copolymerize



r

= r

= 1

 F_A = f_A (diagonal line)

 random copolymer

 rare; A & B of very similar structure



r

>>1, r

>> 1

 block(y) copolymer

 blockiness up as r_A, r_B up

 for coordination, not for radical

Ch 9 Sl 7



r

>1, r

< 1

 special case: r_Ar_B = 1

 ‘ideal copolym’n’

 monomer reactivity depends not on * but on monomer

 as ∆r up, one far more selective

 Most ionic copolym’n is ideal copolym’n ~ not popular

 r_Ar_B ≠ 1 (usually r_Ar_B < 1)

 skewed to more reactive M

 as ∆r up, blockiness up

 conversion dependent ~ A to B

Ch 9 Sl 8



r

r

< 1 or r

r

> 1

 curve intersects diagonal line

 F_A = f_A at that point

 azeotropic copolym’n

 not easy to get when ∆r is large

 extreme case: r_A ≈ r_B ≈ 0

 alternating copolymer

 F_A = 0.5

 r_A & r_B > 1 is rare

 many r_A ^& r_B < 1 systems

 alternating tendency up as r down

Ch 9 Sl 9

(Copolymer) composition drift



Copolymer compos’n changes [drifts] with conversion.

 copolymer composition eqn is for instantaneous f

 F_A ≠ f_A

 one monomer preferentially consumed



to minimize drift [for constant F]

 stop at low conversion

 monomer recycled

 continuous feeding monomer of larger r

 ‘starve-feeding’

Ch 9 Sl 10

Evaluation of r



Fineman-Ross method

 x from feed; y from analysis of copolymer at low conversion

 one set of data gives a point

 least square fitting to a line



Kelen-Tudos method

Ch 9 Sl 11

Radical copolym’n



many commercial copolymers

 SBR, SAN, (ABS (graft)), EVA, ---



Most belong to either

 r_A > 1, r_B < 1

 r_A ^& r_B < 1

Table 9.1 p212

Ch 9 Sl 12

r in radical copolym’n



Reactivity of monomer and radical depends on substituent.

 resonance, polar, (and steric) effects



resonance effect

 A with stabilizing subs, B with less stabilizing subs

 eg, A = ST and B = VAc

 k_BA > k_BB > k_AA > k_AB

 k_BB > k_AA ~ resonance effect larger for radical than for monomer

 k_p of VAc larger than k_p of ST (in homopolym’ns)!

 r_A >1, r_B <1 if resonance only

reactive monomer reactive radical

Ch 9 Sl 13



resonance effect (cont’d)

 Copolym’n is facile for pairs with small ∆r.

 both with stabilizing subs or both with less stabilizing subs

 As ∆r increases

 more blocky structure

 hard to get copolymer with both components

 If too large, no copolym’n

 ST is an inhibitor for homopolym’n of VAc!

Ch 9 Sl 14



steric effect

Odian p496 A

B

k

- do not homopolymerize, but do copolymerize - low reactivity due to steric effect

- trans radical more stable (transition state) competition betw resonance and steric effect

VDC more reactive M

Ch 9 Sl 15



polar effect

 --CH₂CH(W) + CH₂=CH(W)  k_AA --CH₂CH(W) + CH₂=CH(D)  k_AB

 r_A = k_AA/k_AB < 1 and r_B < 1  r_Ar_B << 1

 determines alternating tendency

 alternating tendency up, as r_Ar_B down to 0

 stilbene and MA

 do not homopolymerize;

large steric hindrance in copolym’n

Ch 9 Sl 16

Q-e scheme



rate constant for p and m monomer

 P, Q ~ reactivity ~ resonance effect

 e ~ electrostatic charge ~ polar effect

 setting Q = 1.0 and e= –.8 for ST

 with experiments with ST and others



large Q ~ large resonance ~ reactive M

 large ∆Q  blocky



large e ~ large e withdrawer

 large ∆e  small r_Ar_B  alternating

Table 9.2 p214

Ch 9 Sl 17

Living radical copolym’n



living  All the chains have the same composition and sequence distribution.



statistical only when r

≈ r

≈ 1



If not, composition drift in a chain

 no statistical new chain

 gradient [tapered] copolymer



r the same to normal radical copolym’n?

 should be, but not really

 affected by type of end-capping

 more tapered for ST copolym’n

Ch 9 Sl 18

Ionic copolym’n



r the same to radical copolym’n?



hard to get copolymers with both components

 large ∆r  larger effect of substituent



r depends greatly on solvent and counter-ion



Cationic copolym’n of isobutylene and isoprene is the only

commercial practice.

Ch 9 Sl 19

ZN copolym’n



heterogeneous ‘multi-site’ catalyst

 r observed is the average

 each site has different activity and stereoselectivity



used in EPDM rubber and LLDPE

 EPDM ~ ethylene propylene (non-conjugated) diene monomer

 large ∆r ~ r_ethylene > 50 and r_1-butene < .1

 higher r_α-olefin for smaller subs

 r depends on catalyst

 higher r_ethylene for Ti catalysts

Ch 9 Sl 20

Metallocene copolym’n



homogeneous ‘single-site’ catalyst

 one r and CCE applicable



more comonomer pairs are possible



in copolym’n of ethylene or propylene with α-olefin

 higher content (large r), uniform (inter and intra) distribution of α-olefin

 better mechanical property with less content

 narrower MMD

 beneficial to rheology? ~ wrong

Ch 9 Sl 21

Segmented alternating block copolymers



poly(ester-ether) ~ polyester TPE

 step polym’n of + + +

 a thermoplastic elastomer (TPE)

 a segmented (block) copolymer

 polyether block ~ flexible ~ ‘soft segment’

 polyester block ~ crystallizable ~ ‘hard segment’

 behaves as rubber but thermoplastic

 physical crosslinking

Fig 18.13 p462

Ch 9 Sl 22



segmented PU ~ thermoplastic PU [TPU]

 stiff to flexible ~ dep on soft segment ~ diverse applications



step polym’n of functionalized diblock copolymer

 from living anionic ~ controllable and uniform block lengths

Ch 9 Sl 23

Block copolymer via anionic living



using one-end living chain

 Bu-AA---AA^(-) Li⁽⁺⁾ + n B  Bu-AA---AA-BB---BB^(-) Li⁽⁺⁾

 AB (di)block copolymer, ABC triblock copolymer



using two-end living chain

 Na^{(+) (-)}AA---CH₂CH₂---AA^(-) Na⁽⁺⁾ + n B  Na^{(+) (-)}BB---BB-AA---AA-BB---BB^(-) Na⁽⁺⁾

 Li^{(+) (-)}AA---R---AA^(-) Li⁽⁺⁾ + n B 

Li^{(+) (-)}BB---BB-AA---AA-BB---BB^(-) Li⁽⁺⁾

 ABA triblock copolymer

Ch 9 Sl 24



B must be of higher reactivity (better e

-withdrawing)

 A=MMA and B=ST  PMMA only

 pK_a(toluene) ~ 43; pK_a(ethyl acetate) ~ 30

 A=ST and B=MMA  poly(ST-b-MMA)

 Usually, small amount of CH₂=CPh₂ added

 before MMA addition

 to prevent side reaction

p141

MMA

OCH₃

Ch 9 Sl 25



SIS (or SBS)

 linking two SI (or SB) living chains in nonpolar solvent

 using two-end living IP (or BD) chain in polar solvent

Table 9.3 p215 r_A, r_B

when A = ST

Ch 9 Sl 26



SIS (or SBS) (cont’d)

 sequential addition of ST, IP (or BD), and ST

 ST and IP (BD) are of similar reactivity (pK_a ≈ 43).

 anionic copolym’n of IP (BD) and ST

in the presence of living ST chain ~ commercial (Kraton^®)

 in nonpolar solvent (with Li)

 1,4 > 1,2

 a TPE (or HIPS*)

* HIPS, actually, is a graft copolymer.

See p151

Ch 9 Sl 27

Block copolymer by LRP



Compared to anionic;

 for more diverse monomers

 with less stringent rxn condition

 no control of stereochemistry



NMP

 for ST and some acrylates

 not for methacrylates

 order of addition dep on the catalyst

 more reactive monomer later

 if using TIPNO, Bu-acrylate first then ST

Ch 9 Sl 28



ATRP

 for more monomers than NMP and anionic

 especially methacrylates

 acidic H should be ionized or protected

p141

Ch 9 Sl 29



ATRP (cont’d)

 order of addition

 reactivity of M-X ~ AN > methacrylates > ST ≈ acrylates

 order of K, not of monomer reactivity (p222 wrong)

 copolym’n in a family preferable; eg, -COOMe and -COOBu

 sequential addition gives AB, ABA, ABC, ---

 monomer with higher K first

 ---MMA-Br + ST  PMMA-b-PS

 ---ST-Br + MMA  PS + PS-b-PMMA

 Initiation of MMA is slower than propagation of MMA.

 if ST-MMA sequence necessary, use ‘halogen exchange’

 ---ST-Br + MMA with CuCl

 C–Br > C–Cl

 Initiation of MMA is faster than propagation of MMA.

Fig 9.2 p223

Ch 9 Sl 30



RAFT

 for more versatile monomers

 order of addition

 1st monomer must be of better-leaving radical

 same order as in ATRP? not sure

 seems to be not that critical

 switchable RAFT agent

 more reactive RAFT agent for more reactive monomer



Other living polym’ns can also be used.

 living cationic, GT, ZN, metallocene, RO p223

Ch 9 Sl 31



convert end-group of a (commercial) polymer to initiating functional group

Block copolymer by tandem approach

Ch 9 Sl 32



transform living end to initiating functional group



using dual-function initiator

Ch 9 Sl 33

Block copolymer by coupling



general methods

 step polym’n of functionalized (co)polymers

 linking living chain ends (with X-R-X)

 hard to be complete and give contamination

 due to low conc’n of functional groups

 contamination like homopolymers has to be removed

 often not possible

Ch 9 Sl 34



using ‘click chemistry’

 fast, high selectivity, high yield, no side rxn, ---

 Huisgen cycloaddition

 converting end-group of polymer, or

 using initiator or terminator of living polym’n

Ch 9 Sl 35

Non-linear block copolymer



using multi-functional initiator



linking block copolymer with multi-functional reagent

Ch 9 Sl 36

Graft copolymer synthesis



‘grafting from’ backbone

 direct formation of radical on the backbone

 enhanced by copolym’n with small portion of reactive group (like –CH₃ or = (diene))

 simple and versatile, but not controlled and contamination

∆ or hν

Ch 9 Sl 37

Graft copolymer synthesis



‘grafting from’ backbone (cont’d)

 using LRP

Ch 9 Sl 38



using macro(mono)mer

 by converting chain-end to =

 by catalytic chain transfer p80

 random, tapered, or block sequence

 depending on A and B

Ch 9 Sl 39



‘grafting onto’ backbone

 copolymerization of backbone with

 and ‘grafting onto’

Ch 9 Sl 40

Why block (and graft) copolymer?



TPE

 SBS, polyester TPE, TPU



template for

 functional materials

 nanocomposites

 lithography



drug delivery

 stimuli-sensitive block